Embodiments of the invention relate generally to image sensors. More specifically, embodiments of the invention relate to increasing the signal to noise ratio (SNR) of image sensors using variable exposure techniques.
Selecting the proper exposure duration for image sensors, such as CMOS image sensors (CIS), can be difficult. If the selected exposure duration is too long, pixels may become saturated and the resulting image quality may be poor. If the selected exposure duration is too short, pixels values may be below the dynamic threshold and detail may be lost.
U.S. Pat. No. 5,144,442 (the '442 patent) discloses a method to increase the dynamic range of still images (and of video streams) by acquiring the same scene with multiple exposure periods, then merging the multiple images into a single wide dynamic range image. Conventional techniques to obtain multiple images of the same scene include: using multiple image sensors; and using two sequential image acquisitions, one with a long exposure and one with a short exposure. The first method is expensive, not only because of the need for two image sensors, but also because the two image sensors need to be optically aligned with great precision so that the image of any object in front of the lens will be projected on the same pixel row and column of both image sensors. The second method, using sequential image acquisitions, is cheaper. Because the two acquisitions are not done at the same time, however, the resulting image is susceptible to motion artifacts. Other conventional techniques (e.g. U.S. Pat. No. 5,959,696) offer means to correct for such motion artifacts, but those methods are complex and expensive.
In view of the foregoing, improved methods are needed to increase the dynamic range of image sensors.